Postfix
List queued mails
mailq
Output looks like this:
66ECC20DE6 7332 Mon Jun 4 10:55:35 dodo at bibo.com
(connect to bilbo.beulin.de[172.17.1.40]: Connection timed out)
info at flameboy.de
The value 66ECC20DE6 is the queue id of the eMail.
Look at the content of a mail
postcat -d 66ECC20DE6
Delete a mail
postsuper -d 66ECC20DE6
Delete all mails:
postsuper -d ALL